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Usage Note
Deletes perform normal DML.
That is, they take locks on rows, they generate redo (lots of it), and they require segments in the UNDO tablespace. Deletes clear records out of blocks carefully.
If a mistake is made a rollback can be issued to restore the records prior to a commit. A delete does not relinquish segment space thus a table in which all records have been deleted retains all of its original blocks.
Truncates are DDL and, in a sense, cheat. A truncate moves the High Water Mark of the table back to zero. No row-level locks are taken, no redo or rollback is generated.
All extents bar the initial are de-allocated from the table (if you have MINEXTENTS set to anything other than 1, then that number of extents is retained rather than just the initial). By re-positioning the high water mark,
they prevent reading of any table data, so they have the same effect as a delete, but without all the overhead. Just one slight problem: a truncate is a DDL command, so you can't roll it back if you decide you made a mistake.
(It's also true that you can't selectively truncate -no "WHERE" clause is permitted, unlike with deletes, of course).
By resetting the High Water Mark, the truncate prevents reading of any table's data, so they it has the same effect as a delete, but without the overhead.
There is, however, one aspect of a Truncate that must be kept in mind. Because a Truncate is DDL it issues a COMMIT before it acts and another COMMIT afterward so no rollback of the transaction is possible.
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data1');
INSERT INTO demo_list_list VALUES (2, 1, 'Test Data2');
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data3');
INSERT INTO demo_list_list VALUES (2, 2, 'Test Data4');
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data5');
col high_value format a50
SELECT partition_name, subpartition_name, high_value
FROM user_tab_subpartitions
WHERE table_name = 'DEMO_LIST_LIST';
SELECT COUNT(*) FROM demo_list_list SUBPARTITION(BOEING_SUNDAY);
ALTER TABLE demo_list_list TRUNCATE SUBPARTITION boeing_sunday;
SELECT COUNT(*) FROM demo_list_list SUBPARTITION(BOEING_SUNDAY);
Truncating a parent table partition truncates the corresponding child table partition
ALTER TABLE <parent_table_name> TRUNCATE PARTITION <partition_name> CASCADE;
CREATE TABLE ref_parent (
table_name VARCHAR2(30),
order_date DATE,
num_rows NUMBER)
PARTITION BY RANGE(num_rows) (
PARTITION num_rows1 VALUES LESS THAN (100),
PARTITION num_rows2 VALUES LESS THAN (1000),
PARTITION num_rows3 VALUES LESS THAN (MAXVALUE));
ALTER TABLE ref_parent
ADD CONSTRAINT pk_ref_parent
PRIMARY KEY (table_name)
USING INDEX;
CREATE TABLE ref_child (
table_name VARCHAR2(30) NOT NULL,
index_name VARCHAR2(30) NOT NULL,
CONSTRAINT fk_ref_child_parent
FOREIGN KEY(table_name) REFERENCES ref_parent(table_name) ON DELETE CASCADE)
PARTITION BY REFERENCE(fk_ref_child_parent);
INSERT INTO ref_parent VALUES ('OBJ$', SYSDATE-10, 42);
INSERT INTO ref_parent VALUES ('TAB$', SYSDATE-5, 200);
INSERT INTO ref_parent VALUES ('COL$', SYSDATE+5, 1111);
INSERT INTO ref_parent VALUES ('IND$', SYSDATE+10, 9999);
INSERT INTO ref_child VALUES ('OBJ$', 'PK_OBJ$');
INSERT INTO ref_child VALUES ('TAB$', 'PK_TAB$');
INSERT INTO ref_child VALUES ('COL$', 'PK_COL$');
INSERT INTO ref_child VALUES ('COL$', 'UI_COL$');
INSERT INTO ref_child VALUES ('IND$', 'PK_IND$');
COMMIT;
SELECT * FROM ref_parent;
SELECT * FROM ref_child;
ALTER TABLE ref_parent TRUNCATE PARTITION num_rows2 CASCADE;
CREATE INDEX idx_sc_srvr_id ON CLUSTER uw_cluster;
INSERT INTO cservers (srvr_id, network_id) VALUES (1,2);
INSERT INTO cserv_inst (custacct_id, srvr_id, ws_id) VALUES ('TEST', 1, 4);
TRUNCATE CLUSTER uw_cluster;
Truncate Materialized View Log
Truncating materialized view and materialized view logs. Read the docs before doing this as there are restrictions on what can and can not be preserved.
This procedure was provided to the library by JP Vijaykumar
/* Table owners can always truncate their own tables but for a third party to truncate a table that
username must be granted escallated privileges such as the DROP ANY TABLE system privilege or a role such as DBA that posses that privilege.
Granting escallated privileges should never be done for routine activities so it is recommended that a procedure such as this one be
placed into schemas where truncates may be required and EXECUTE privileges on the procedure be granted as is appropriate. */
conn uwclass/uwclass@pdbdev
CREATE TABLE t1 AS
SELECT * FROM all_tables;
CREATE OR REPLACE PROCEDURE trunc_tab(p_tabname IN VARCHAR2) AUTHID DEFINER AS
/**********************************
AUTHOR JP Vijaykumar
ORACLE DBA
**********************************/
v_num NUMBER(10):=0;
v_owner VARCHAR2(30);
v_user VARCHAR2(30);
sql_stmt VARCHAR2(2000);
BEGIN
SELECT username
INTO v_owner
FROM user_users;
SELECT sys_context('USERENV','SESSION_USER')
INTO v_user
FROM dual;
IF (v_owner = v_user) THEN
execute immediate sql_stmt;
ELSE
SELECT COUNT(*)
INTO v_num
FROM all_tab_privs
WHERE table_name = UPPER(p_tabname)
AND table_schema = v_owner
AND grantee = v_user
AND privilege in 'DELETE';
IF (v_num > 0) THEN
execute immediate sql_stmt;
ELSE
RAISE_APPLICATION_ERROR(-20001,'Insufficient privileges.');
END IF;
END IF;
EXCEPTION
WHEN OTHERS THEN
RAISE_APPLICATION_ERROR(-20001,'Insufficient privileges');
END trunc_tab;
/
